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Classical' Age-Modelling of Radiocarbon Sequences

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Classical' Age-Modelling of Radiocarbon Sequences ARTICLE IN PRESS Quaternary Geochronology xxx (2010) 1e7 Contents lists available at ScienceDirect Quaternary Geochronology journal homepage: www.elsevier.com/locate/quageo Research Paper Methods and code for ‘classical’ age-modelling of radiocarbon sequences Maarten Blaauw* School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, United Kingdom article info abstract Article history: Ageedepth models form the backbone of most palaeoenvironmental studies. However, procedures for Received 19 February 2009 constructing chronologies vary between studies, they are usually not explained sufficiently, and some are Received in revised form inadequate for handling calibrated radiocarbon dates. An alternative method based on importance 22 October 2009 sampling through calibrated dates is proposed. Dedicated R code is presented which works with calibrated Accepted 27 January 2010 radiocarbon as well as other dates, and provides a simple, systematic, transparent, documented and Available online xxx customizable alternative. The code automatically produces ageedepth models, enabling exploration of the impacts of different assumptions (e.g., model type, hiatuses, age offsets, outliers, and extrapolation). Keywords: Ó Radiocarbon age-modelling 2010 Elsevier B.V. All rights reserved. Calibration Software Exploratory analysis 1. Introduction “classical” ageedepth modelling such as linear interpolation (31) or linear/polynomial regression (18). Ageedepth models are built to estimate the calendar ages of As will be discussed below, calibrated 14C dates often have irreg- depths in a core from a deposit, based on limited numbers of dated ular distributions which cannot be properly reduced to single point depths and on assumptions as to how the deposit has accumulated estimates with symmetric error bars. However, most age-modelling between those dated depths. Given the intensive use of ageedepth routines do indeed simplify calibrated distributions to symmetric modelling, surprisingly few details have been published about the ones. Bennett (1994) and Bennett and Fuller (2002) discussed basic techniques involved, adequate software seems to be largely missing ageedepth models based on linear interpolation, polynomial and or under-used, and details of applied ageedepth modelling spline regression, but based on uncalibrated radiocarbon dates. Proxy assumptions are often missing in the literature. graphing software Tilia (Grimm, 1990)producesageedepth models, A review of all papers published in 2008 in the journals Quater- but i) does not calibrate dates, ii) does not take into account errors, nary Geochronology, Quaternary Science Reviews, Journal of and iii)doesnotprovideageedepth graphs. Heegaard et al. (2005) Quaternary Science and The Holocene, indicates 93 papers applying applied mixed-effects modelling, but assumed that calibrated 14C some kind of ageedepth model through fossil proxy deposits ages can be approximated by a simpler statistical distribution (e.g., (Table 1). Of these papers, all 82 that used 14C dates provide details Gaussian). It is likely that studies which do not report their about their calibration procedures (e.g., which calibration curve and ageedepth modelling software often use regression or spline func- software were applied). However, often much less information is tions which generally cannot deal with irregular distributions. supplied about subsequent ageedepth modelling (regularly this Whereas proxy graphic software psimpoll (Bennett,1994) can handle information can only be gleaned from graphs). Sixty papers do not tell calibrated 14C dates, they need to be calibrated outside the software. which (if any) point estimates were used, 71 do not mention the Although Bayesian methods can produce very reliable ageedepth software used to produce ageedepth curves, 65 do not indicate models (e.g., Blaauw and Christen, 2005; Bronk Ramsey, 2008), whether ageedepth uncertainties were calculated, and 18 do not Table 1 shows that by far most studies continue to use more basic, specify the applied ageedepth model. Eleven papers apply some ‘classical’ age-depth models (especially for sites with only a handful form of Bayesian ageedepth modelling, whereas the remainder use of dates). Here these classical approaches will be discussed, and a technique based on probability sampling methods will be proposed in order to take into account the multi-modal and asymmetric nature of calibrated 14C dates. A more systematic and transparent approach e * Tel.: þ44 28 9097 3895. to produce classical age depth models is presented, based on free E-mail address: [email protected] and open-source software. 1871-1014/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2010.01.002 Please cite this article in press as: Blaauw, M., Methods and code for ‘classical’ age-modelling of radiocarbon sequences, Quaternary Geochronology (2010), doi:10.1016/j.quageo.2010.01.002 ARTICLE IN PRESS 2 M. Blaauw / Quaternary Geochronology xxx (2010) 1e7 Table 1 Literature analysis of primary literature reporting ageedepth models, published in 2008 in Quaternary Geochronology (2 papers), Quaternary Science Reviews (40), Quaternary Research (10), Journal of Quaternary Science (13) and The Holocene (28). Publications citing previously published ageedepth models were not taken into account. As several papers applied a number of ageedepth models and types of dates, the numbers do not always add up. 17 papers mentioned the removal of dates identified as outlying. Dates Point estimate Model Model error Age-model software 14C (82) Not specified (60) Linear interpolation (31) Not specified (65) Not specified (71) Tephra (11) Full distribution (13) Not specified (18) 2 sd error (17) Oxcal (6) 210Pb/Cs (9) Mid (5) Linear regression (13) 1 sd error (6) Bpeat (4) U/Th (8) Median (4) Bayesian (11) Mixed-effect (3) OSL (5) Intercept (3) Linear regression (5) Bchron (1) Tuning (4) Mean (1) Spline (4) psimpoll þ BCal (1) Varves (2) Weighted mean (1) Mixed-effect (3) Other (2) Mid of most probable range (1) CRS (2) Other (2) 2. Radiocarbon calibration yjqwN mðqÞ; s2 (1) 14 The importance of using calibrated C dates cannot be overstated. where j means ‘given’,ands is a combination of the measure- 14 Owing to past changes in atmospheric Cconcentration,therela- ment uncertaintyqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi and that of the calibration curve at the calendar 14 tionship between C and calendar age is far from linear, thus age, s(q), s ¼ sd2 þ sðqÞ2. Usually the total probability over a large requiring a calibration curve (Reimer et al., 2009). Whereas an range of calendar years is normalised to 100%. Because the resolution 14 uncalibrated C date can be assumed to have symmetrical errors of most calibration curves is decadal, calibration at yearly intervals (normal distribution), calibration will often result in widened, requires (usually linear) interpolation of m(q) and s(q). asymmetrical and multi-peaked calendar age uncertainties (Fig.1;all The currently internationally accepted calibration curves are calendar ages are expressed in years before AD 1950 or cal BP, unless IntCal09 (northern hemisphere atmospheric; Reimer et al., 2009), mentioned otherwise). These asymmetrical distributions can cause Marine09 (for marine dates; Reimer et al., 2009), and SHCal04 problems when applying classical interpolation/regression tech- (southern hemisphere atmospheric; McCormac et al., 2004). niques, as they often assume symmetrically/normally distributed Alternative curves exist, e.g., for dating organic material deposited errors. after nuclear weapons testing (Hua and Barbetti, 2004; Levin et al., 14 Standard C calibration software generally applies ‘probabilistic 2008). 14 calibration’ to provide calendar age estimates of C dates (Stuiver In case any of the 14C dates in a sequence is suspected to be prone and Reimer, 1989, 1993; van der Plicht and Mook, 1989). In this to a systematic age-offset (e.g., owing to a lake hard-water effect, or 14 method, a C date with reported mean and error y Æ sd, is assumed to an additional local reservoir effect for marine data; see Reimer and 14 to have a Gaussian distribution on the C age scale; y w N(y, sd). Reimer, 2001, http://calib.org/marine/), this should be corrected for 14 Whereas fluctuations in the calibration curve often cause single C before calibrating. If an uncertainty estimate of the reservoir effect is ages to correspond to multiple calendar ages, every calendar age 14 available,qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the error of the C date should be increased accordingly will always have only one single corresponding 14C age. Therefore, (sd ¼ sd2 þ sd2 ,wheresd is the error of the reservoir effect). to calibrate a 14C date, we aim to find the probability of that date res m res belonging to a given calendar year. For a range of calendar years q, Calibrated distributions are usually reduced to 1 or 2 sd calibrated the corresponding 14C age of the calibration curve, m(q), is looked up ranges, taking into account their asymmetric and multi-peaked and compared with the 14C measurement (see Christen, 1994): shapes. In standard calibration software such as CALIB (Stuiver et al., 2005), these ranges are obtained by calculating the highest posterior density (hpd) ranges: i) probability distributions are nor- malised to 100% (Eq. (1)), ii) the calendar years are ranked according to their probabilities, iii) those calendar ages with a cumulative summed probability at or below the 1 sd (c. 68%) or 2 sd (c. 95%) threshold
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